Some audiophiles prefer high sensitivity full range speakers for use with low power amps, usually single ended triode tube designs. I've built several versions of this kind of speaker using Fostex, Audio Nirvana, and Tang Band drivers, always with the same result: Used in a reasonably sized bass reflex or MLTL enclosure they all lack bass extension and impact. A simple LR baffle step compensation filter can enhance the bass response, but only at the expense of several dB of sensitivity. Your 96dB sensitivity driver becomes a 90 to 92dB sensitivity driver. The result has typically been a very compelling sound, but with too little sensitivity to achieve room-filling levels of dynamic music with a 3.5 watt 2A3 amp or even an 8 watt 300B amp. The amp runs out of steam, and the music peaks sound congested and opaque. So this is a great combination if your musical preference is limited to acoustic music or classical chamber music, but not if you also want to hear rock music or full orchestra classical. It's just Hoffman's Iron Law in action: Small size, high sensitivity, deep powerful bass - choose any two.
The object of the exercise with these speakers was to overcome these limitations, and to accomplish this with an affordable open back design. The drivers I chose were the 8" Dayton Audio PS220-8 full range driver, mounted on a simple open back baffle at ear-level (35”) above a 15" H-frame Eminence Alpha 15A, driven by its own 100 watt subwoofer plate amp. The measured and subjective results are good. Full range response from 37hz to 20khz, with no significant peaks and no gradually rising treble response.
The only fly in the ointment with the Dayton driver is a fairly broad 5dB peak centered at 3.6khz. Without a notch filter this results in a fairly bright in-your-face sound. As much as I would have liked to remain pure to the conservative single driver ideology and wire the driver directly to the amp with no series components, I found the system sounds much better with a parallel notch filter to take the peak down by about 4dB. This filter consists of a 0.2mH air core inductor, a 12.5 ohm resistor, and a 9uF metalized poly cap all installed in series with the positive speaker terminal. With the notch filter in place the open back driver provides smooth response from about 200hz to 20khz. There’s some brightness in the 10khz to 20khz range on axis, but this smooths out when you listen about 10-15 degrees off axis.
The Eminence driver is mounted in a Martin King design H-frame whose internal dimensions are 16”X16”X16”. Design information can be found here:
http://www.quarter-wave.com/OBs/U_and_H_Frames.pdfThe plate amp is connected to the main amplifier’s speaker outputs, with the crossover set at the maximum 180hz, providing a good continuity between the Dayton driver’s 200hz F3 and the Eminence driver’s high frequency rolloff.
This combination can produce reasonably loud sound with my 3.5 watt 2A3 SET amp or with my OddWatt Audio PoddWatt class A push pull amp. However, I’ve found it requires more power to reach its full potential. An 8 watt 300B amp is adequate. It really shines with a pair of 25watt OddWatt OddBlock class A KT88 amps. Of course, it also sounds very good with a 65 watt Cambridge Audio integrated sand amp.
I’ll be demonsrating the speakers at the Lone Star Audio Fest in Dallas on the first weekend in May. The cost of all the parts needed to build your own diy pair, including two 100 watt Dayton Audio subwoofer plate amps, is about $600 from Parts Express. This does not include the cost of materials for the “enclosure”, which consists of a simple square box for each subwoofer with a flat baffle for the Dayton driver mounted on top of the sub box. The side panels in the picture are for aesthetic purposes only and do not affect the sound. If the WAF factor isn't important to you, a couple of shelf braces could be substituted.
Some of my past projects along with my commercially made speakers can be seen here:
http://fredt300b.smugmug.com/Hobbies/Sp ... &k=2cx37hF
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